Technical Field
Systems and methods for measuring current in a conductor.
Description of the Related Art
Multimeters, also called digital multimeters or “DMMs,” are adapted for measuring a number of parameters generally needed for service, troubleshooting, and maintenance applications. Such parameters typically include a.c. (alternating current) voltage and current, d.c. (direct current) voltage and current, and resistance or continuity. Other parameters, such as frequency, capacitance, and temperature, may also be measured to meet the requirements of the particular application. In order to measure current with a general purpose multimeter, an internal current shunt having a known resistance must be inserted in the current path, requiring a break in the current-carrying conductor. The voltage drop across the current shunt is then measured to determine the current flow.
General purpose multimeters employing internal current shunts are generally limited to ten amperes maximum because of the capacity of the multimeter test leads and circuitry to carry the current. Furthermore, the multimeter generally must be protected with an internal fuse to prevent excessive current levels from flowing through the multimeter, both for safety reasons and to prevent damage to the multimeter. The difficulty in removing a blown fuse, coupled with the time and cost necessary to procure a replacement fuse, make it desirable to obtain a non-contact current measuring instrument that requires no internal fuse.
Clamp-on multimeters provide improved capability for measuring current over general purpose multimeters by employing an integral current clamp which senses the current in the current-carrying conductor without having to cut the current-carrying conductor or break the circuit including the current-carrying conductor. A current clamp is typically provided in the same housing with a multimeter which measures other parameters such as voltage and resistance in the conventional manner using separate test probes. The current clamp is closed around the current-carrying conductor, which may include copper wires and buss bars, to sense the magnetic field created by the current flow. The current clamp provides a voltage signal for measurement by the multimeter which calculates and displays the measured current level. Because there is no current shunted from the current-carrying conductor through the clamp-on multimeter, the constraint on the maximum current that may be measured has largely been eliminated. Likewise, the internal fuse has been eliminated in clamp-on multimeters.
In order to obtain a valid current measurement, the magnetic core in the current clamp must completely encircle the current-carrying conductor so that the current clamp is completely closed. The current clamp must be mechanically actuated to open the jaws, the current-carrying conductor inserted, and the jaws then closed around the current-carrying conductor. In tight physical spaces such as an electrical cabinet, inserting the clamp-on multimeter and using this technique to make a current measurement is inconvenient and difficult. Moreover, the jaws must be aligned to complete the magnetic core for obtaining a valid current measurement. Clamp-on multimeters are therefore difficult to use in confined spaces and require a large physical space in which to open the jaws of the current clamp.
Clamp-on multimeters also tend to be physically heavy because of the substantial amount of iron used on the magnetic core. Furthermore, high levels of current may saturate the magnetic core. The current measuring capacity of the clamp-on multimeter is accordingly limited to current levels that do not saturate the magnetic core. The clamp-on multimeters and wired Rogowski coil are both able to sense alternating current flowing through a conductor surrounded by the clamp or Rogowski coil. There are, however, a number of differences between the Rogowski coil and the clamp. For example, a Rogowski coil is more flexible and has a smaller cross-section than the substantially rigid clamp of the multimeter. The Rogowski coil can accordingly be used in confined spaces that are too tight and/or too small for the clamp-type multimeter. Further, the loop of a Rogowski coil can be reshaped to surround conductors having cross-sections that the clamp cannot close around. Another difference is the greater current measuring capability of the Rogowski coil as compared to the clamp. Specifically, an air core does not become saturated at levels of current that saturate the magnetic material of the cores of the clamp.
However, the Rogowski coil is limited by the parasitic resistance, capacitance and inductance of the connecting cable. To minimize parasitic effects, it is desired to keep the length of the connecting cable as short as practical. The length of the connecting cable also limits the distance between the placement of the Rogowski coil and the position of a technician viewing the measurements taken with the coil. The conductor enclosed by the Rogowski coil may be in a room or cabinet with limited room for the person operating the multimeter. The conductor may carry large voltage and currents that represent a danger to the technician because accidental contact with an exposed conductor could be harmful and possibly fatal. Despite the progress made by systems such as those shown and described in U.S. Pat. No. 8,330,449, there remains an unmet need for improved convenience and safety that cannot be provided by a Rogowski coil wired to a multimeter.